ZnSe is one of the II-VI compound semiconductors which find extensive use as component materials of optical elements because their band gap ranges widely from 3.5 eV for ZnS to 0.1 eV for CdHgTe. In particular, ZnSe has a great potential for practical use as a material for making LEDs that emit light in the blue region of the spectrum. Light-emitting diodes (LED) capable of light emission in the infrared region or in the red, orange, yellow and green regions have also been fabricated and used commercially in high volumes. However, blue light emitting LEDs have not yet reached a commercial stage, chiefly because of the scarcity of suitable materials.
General considerations that are taken into account in selecting suitable materials for LEDs are as follows:
(i) direct-gap semiconductors are desirable;
(ii) ease of p-n junction formation;
(iii) ease of obtaining a large-area substrate crystal; and
(iv) simple device process.
Besides these considerations, materials suitable for making LEDs that emit in the blue region must satisfy the additional requirement that they have a band gap of 2.5 eV or more.
Several semiconductor materials are known to meet these conditions and they include GaN, SiC, ZnSe, ZnS, GaAlN, and ZnSSe. However, all of these materials are still at the laboratory stage and no commercially important materials have yet been attained.
In the absence of a good quality GaN substrate, it has been proposed that a GaN LED be fabricated by heteroepitaxial growth of n-GaN and i-GaN on a sapphire substrate. Because of the hetero-epitaxy involved, however, this product suffers the disadvantage that cracks are prone to develop in the crystalline layers due to thermal expansion mismatch and, therefore, that the product deteriorates rapidly. A further defect is that the i-layer is difficult to control in such a way as to avoid fluctuations in the operating voltage.
Although an SiC LED uses an indirect-gap semiconductor and is unable to emit light with high efficiency, bulk crystals of SiC are known and can be used as the substrate of the device. Since the crystal growth involved is homo-epitaxy, no problems occur due to the difference between lattice constants. Notwithstanding these advantages, it is difficult to prepare a SiC substrate of good quality, and manufacture of SiC LEDs is far from being at a commercial stage.
ZnSe is a direct-gap semiconductor having a band gap (Eg) of 2.7 eV, but it has the disadvantage that only an n-type crystal can be produced. After many years of failure to produce p-type crystals of ZnSe, a report has recently been published that announces a successful growth of p-ZnSe by doping with Li. This direction represents the future of ZnSe LEDs (J. Nishizawa et al., "Blue light emission from ZnSe p-n junctions", J. Appl. Phys., Vol. 57, No. 6, pp. 2210-2216 (1985)).
The usual practice of making an LED from ZnSe involves hetero-epitaxy of ZnSe on a GaAs substrate because a ZnSe substrate of good quality is difficult to produce. On the other hand, a GaAs substrate of good quality is available, and a ZnSe epitaxial layer can be grown on this substrate by a conventional technique such as molecular-beam epitaxy or pyrolysis of organic metals. Of course, ZnSe layers are better grown by homo-epitaxy in order to attain a desirable lattice match. To obtain an epitaxially grown layer of good quality, the ZnSe substrate on which it is grown must also be of good quality. In addition, for performing a wafer process in an efficient way, it is strongly desired to prepare a large-area single-crystal ZnSe wafer. However, it is difficult, even using state of the art techniques, to prepare ZnSe single crystals of good quality.
While many crystal-growth methods have been attempted, they have not met with success in producing large single crystals of ZnSe in high yields and with few defects.
One reason for the difficulty involved i making single crystals of II-VI compounds is that these materials will not melt unless a high pressure is applied. As for Zn compounds, the melting point increases in the order of ZnTe, ZnSe and ZnS, with a corresponding increase in pressure required to melt these compounds.
In the case of ZnSe, inert gas pressures in the range of from twenty to thirty or the like to 50 atm must be applied, and a temperature as high as 1,520.degree. C. is required in order to melt ZnSe without causing its sublimation.
The liquid encapsulated Czochralski (LEC) method has been known to be effective in growing single crystals of III-V compound semiconductors in bulk form. In this technique, the melt is covered with a layer of B.sub.2 O.sub.3 and a high inert gas pressure applied to prevent sublimation of a group V element. However, this method is not applicable to II-VI compound semiconductors since the liquid encapsulant will react with the starting materials to be melted.
The inability to use the LEC method is another reason why it is extremely difficult to grow large single crystals of II-VI compound semiconductors in bulk form. As alternatives, the high-pressure Bridgman method, the sublimation method, the iodine transport method, and the solution growth method have been attempted with a view toward growing single crystals of ZnSe, but they all have proved unsatisfactory in one or more aspects including electrical properties, crystallinity, purity, crystallographic shape, size, and crystal growth rate of the resulting crystals.
An overview of the previous attempts at growing single crystals of II-VI compound semiconductors is given hereinafter. For outlines of the conventional growth techniques for II-VI compound semiconductors, see the following references:
Kessho Koqaku Handbook (Crystal Optics Handbook), pp. 699-702 (1971), Kyoritsu Shuppan; Tankessho Sakuseiho (Preparation of Single Crystals), edited by the Physical Society of Japan, pp. 121-149 (1966), Asakura Shoten; Akasaki et al, "Aoiro LED no Shorai Tenbo (Future Aspect on Blue Light-Emitting Diode)", Journal of the Institute of Electronics and Communication Engineers of Japan, vol. 69, No. 5, pp. 487-491 (1986).
(A) High-pressure Bridgman Method
FIG. 4 is a schematic diagram of a typical apparatus used in the high-pressure Bridgman method. The method performs crystal growth from a melt and has the advantage that a large single crystal can be easily grown. However, in the case of II-VI semiconductors, this method involves considerable technical difficulties because of the need to use extremely high temperatures and pressures.
A long vertical rotating crucible 24 having a tapered lower end is charged with a starting ZnSe material which is pressurized with an argon gas 26 to approximately 50 atm while it is melted with a heater 28. The region having a flat temperature profile is used as the liquid 30 from which a crystal is to be grown, with a temperature decreasing portion being formed in the lower part. As the rotating crucible is allowed to descend, the liquid in the tapered section starts to solidify. Although the growing crystal has various crystallographic orientations, one having a certain orientation will grow in predominance over crystals having other orientations, with the result that a single crystal will form as the crucible is passed through the low-temperature zone, first the tapered section, then the flared part, and finally the body of the crucible.
A large-diameter crystal can be formed by the high-pressure Bridgman method, but the resulting crystal contains many defects. Furthermore, the crystal obtained is not necessarily a single crystal.
An alternative technique employing melting at high pressure is the Tammann method in which rather than allowing the crucible to descend, the power of a heater is controlled such that the temperature of the melt is lowered while maintaining a certain temperature gradient.
As with the Bridgman method, a ZnSe single crystal can be drawn from the melt by the Tamman method, but the crystal contains many defects and also sometimes fails to be a purely single crystal.
A common problem to the Bridgman and Tammann methods is that because of the high temperatures employed, ingress of impurities such as Si and C is very likely to occur and single crystals of high purity are difficult to attain.
(B) Solution Growth Method
The principle of the solution growth method is that ZnSe is dissolved in a solution of a solvent such as Bi, Sn, In, Se--As or Se--As--Sb to form a saturated ZnSe solution. The solution is either cooled or provided with a temperature difference such that a desired crystal will form on the cool side.
Nishizawa et al. studied a method of effecting crystallization at 1,050.degree. C. from polycrystalline ZnSe dissolved in a solvent (Zn or Se), with the vapor pressure of Zn or Se being properly controlled. As a result, they found that a stoichiometric ZnSe crystal could be grown by this method. They also showed that this method was capable of forming a p-type ZnSe single crystal (Nishizawa et al., J. Appl. Phys., Vol. 57, No. 6, pp. 2210-2216 (1985)). However, this method which involves crystallization from solution is only capable of producing small single crystals with diverse shapes that defy exact shape control.
(C) Sublimation Method
The II-VI compounds have very high sublimation pressures. The sublimation method advantageously uses this fact in which a starting material is allowed to sublime and a desired crystal is formed in the cool zone by diffusion.
FIG. 5 is a schematic diagram of a typical apparatus used in the sublimation method. A quartz ampule in closed tube form is placed horizontally and heated with a heater in such a way that a starting ZnSe material 34 in portion is heated at 1,015.degree. C. and solid Se or Zn 36 in portion b is held at about 500.degree. C. so as to keep the vapor pressure of Se or Zn at equilibrium. The portion c of the ampule is heated at 1,000.degree. C. Sublimation of ZnSe at portion a is expressed by the following reaction scheme: EQU ZnSe.fwdarw.Zn+1/2Se.sub.2
The resulting gas is cooled at portion to form crystalline ZnSe 32. The portion c is either fitted with a seed crystal or made in a conical form so that the random formation of crystalline nuclei will be prevented to ensure the production of a mass of single crystal.
The major disadvantage of the sublimation method is that its success largely depends on growth conditions and that a polycrystalline form rather than a single crystal of high quality is more likely to occur under certain conditions.
(D) Piper and Polich Method
A modification of the sublimation method that effects sublimation in an Ar gas atmosphere was developed by Piper and Polich. This modified method is called the Piper and Polich method after the names of the inventors.
FIG. 6 is a schematic representation of the apparatus used in the Piper and Polich method.
A sintered ZnSe charge 38 is packed in a long horizontal container. Argon gas 40 is introduced into the container at a pressure of about 1 atm and the container is inserted into a heater 44 having a triangular temperature profile. As the container is moved to the right in FIG. 6, ZnSe is heated to sublime.
The sintered ZnSe charged is heated to sublime as it passes through the zone having a temperature gradient XY. The vapor moves toward the tip of the container where it is crystallized. As a result, a space is formed between the crystal and the sintered material which has a lower density than the crystal. When the container has completed its rightward movement (referring to FIG. 6), a single crystal of ZnSe is left behind in the container. The temperature at point Y is 1,360.degree. C. The temperature gradient of a zone YZ is 20.degree. C./cm or less. For details of the Piper and Polich method, see their article entitled "Vapor-Phase Growth of Single Crystals of II-VI Compounds", J. Appl. Phys. Vol. 32, 1278-1279 (1961). It should be noted here that in the Piper and Polich method, the ZnSe charged is sintered by packing ZnSe powder. This method also has the disadvantage in that its success largely depends on growth conditions and that a polycrystalline form rather than a single crystal of high quality is more likely to be produced under certain conditions.
(E) Halogen Transport Method
The principle of this method is to grow a single crystal 42 by using halogens such as I.sub.2 and Cl.sub.2 as a transporting agent. When I.sub.2 is used, this method is called the iodine transport method.
FIG. 7 shows a typical apparatus used in the iodine transport method. A vertical growth container is charged with a ZnSe powder. The "powder" is actually a sinter prepared by filling an evacuated quartz ampule with a prebaked (900.degree. C.) commercially available ZnSe powder of high purity and firing the same at 1,000.degree. C. for at least 48 hours. This treatment is effective in removing impurities and increasing the diameter of ZnSe particles.
A mixture of the ZnSe powder and iodine is placed in the growth container. The top of the container is closed with a quartz rod that is fitted with a seed crystal 46 of ZnSe on its underside.
The lower part of the container is heated to a higher temperature (850.degree. C.) than the part where the seed crystal is attached (840.degree. C.). The reversible reaction given below describes the process: ##STR1## As shown, ZnSe+I.sub.2 and Se.sub.2 are generated at the high-temperature end (bottom) of the container and are transported upward to reach the cool end where crystallization 48 of the ZnSe occurs. The iodine then diffuses back to the charge and recommences the transport cycle. The ZnSe will not sublime vigorously at temperatures on the order of 850.degree. C., so it is transported with the aid of I.sub.2.
The evacuated container is sealed off without using argon, so it is filled with an iodine atmosphere.
The maximum temperature employed in this method is 850.degree. C., with the temperature gradient being on the order of 20.degree. C./cm. Under these conditions, the free movement of ZnI.sub.2 and Se.sub.2 occurs.
Depending on growth conditions, this method often results in the formation of polycrystalline form, rather than a single crystal. This method also suffers the disadvantage of a very slow growth rate.
(F) CVD Process
The principle of the CVD process is to react the Zn vapor with H.sub.2 Se gas and to allow ZnSe to be deposited on a substrate after it has formed as a result of the following reaction:
H.sub.2 Se (gas)+Zn (vapor).revreaction.ZnSe (solid)+H.sub.2 (gas)
The CVD process can be implemented by several methods.
FIG. 8 shows an apparatus used in one technique of the CVD process. The system consists of a zinc evaporating furnace and a reactor furnace which are placed in one vessel.
The zinc evaporating furnace 11 has a heater 12 that heats the zinc in a zinc container 13 to a molten state 14. Argon is introduced into the evaporator 11. The argon serves as a carrier gas that transports the Zn vapor toward the reactor 15. Also introduced into the reactor 15 is H.sub.2 Se gas.
The reactor 15 is equipped with a heater 16 that heats a substrate 17 to an appropriate crystallization temperature. In a region where the substrate is heated at the crystallization temperature, H.sub.2 Se reacts with the Zn gas to form ZnSe which is deposited on the substrate 17. The unreacted gas is discharged from the reactor 15, with the synthesized ZnSe crystal left behind on the substrate 17. This product, however, is polycrystalline and it is extremely difficult to attain a single crystal of ZnSe by the CVD process.
In another approach, the CVD process can be practiced by using ZnCl.sub.2 in place of molten zinc with argon used as a carrier gas. Hydrogen can also be used as a carrier gas.
The substrate 17 is about 1 m long and may be in the form of a rectangular prism of several centimeters per side. With such a large-area substrate, a thin but large-size polycrystalline ZnSe film can be deposited.
Unlike the other methods described above, the CVD process is unable to permit single-crystal growth. The ZnSe polycrystalline produced by the CVD process finds utility as a window defining member and a lens material in a CO.sub.2 laser since it offers high transmittance of infrared light.
(G) Travelling Heater Method (THM)
The THM is one of the techniques for crystal growth which have been known in the art for many years as described, e.g., in Jpn. J. Appl. Phys., Vo. 17, 1331 (1978); J. Cryst. Growth, Vol. 45, 204 (1978); ibid., Vol. 28, 29 (1975); and J. Electrochem. Soc., Vol. 110, 1150 (1963). In the THM, a heater is moved with respect to a specimen so as to obtain a single crystal while the impurities in the specimen are segregated. Although the THM can operate easily and stably, the THM includes problems in which an extremely high purity is difficult to obtain since a solvent metal is generally used, and a strict temperature control is required since crystal defects are formed by fluctuation of the temperature.
As the foregoing overview of the previous methods shows, many efforts have been made to prepare single-crystal ZnSe through various techniques of crystal growth. However, none of the techniques proposed so far are capable of efficient growth of large single-crystal ZnSe that has very few defects and which is of extremely high purity. These known techniques including the Bridgman, Tammann, and Piper and Polich methods are not capable of effectively preventing ingress of impurities such as Si, Al and C, and hence single crystals of high purity are unattainable. Besides this purity problem, the conventional methods that require high temperatures have a common problem in that the crystals they produce have many defects. Those which permit crystal growth at low temperatures are disadvantageous in that the crystals they produce have low purity and are too small in size.